Default Programs/Set Associations not cooperating

Until recently, all photos in 'My Pictures' albums (and I have many) were viewable at a glance; now they are a slew of undiscernable icons.  I quickly found that none of my .jpg photos would open (Photo Viewer) and tried to change the default program for those in "Set Associations" from Photo Viewer to Live Photo Gallery.  It won't do it, nor will it allow me to reset the default, which is shown as Photo Viewer.   It's absurd to think I must go through hundreds of photos and manually rename them .jpeg in order to view them with Live Photo Gallery.  It's even more ridiculous that I must now manually open any untold number of photos to find the ONE I might be looking for.  How can I get this corrected in order to have my photos readily viewable.  At least please tell me how I can, or why I cannot, reset my default to alleviate the current .jpg vs .jpeg problem.  Thank you!